1. Field of the Invention
The present application generally relates to electrical cables, including selection of materials for their conductive, insulating, or dielectric properties. Specifically, the application is related to fire-resistant co-axial cables with ceramifiable silicone rubber or ceramic fiber dielectric between the conductors and outside of the outer conductor.
2. Description of the Related Art
Since the Sep. 11, 2001 attacks on the World Trade Center and Pentagon, there has been a world-wide emphasis on improving communications during emergencies. In the first minutes of an emergency, communication among civilians and first responders is often through wireless communication devices, such as cellular telephones. While wireless signals, being electromagnetic radiation typically in the radio frequency (RF) range, are impervious to damage and do not depend on wires for transmission, the wireless signals depend on other infrastructure to communicate. This infrastructure includes antennas, switching equipment, towers, repeaters—and wires.
Ground zero of a disaster, man-made or natural, is often localized to a particular geographic area. At least some local cell towers may be operational. But cell phones within large buildings often do not connect directly with cell towers. Such buildings, as well as shopping centers and stadiums, may have too many obstacles and reflections for conventional cell phone-to-tower connections. For example, the metal reflective film applied to glass facades of commercial buildings prevents transmission of RF energy outside the building. Or the buildings may simply be too large for RF signals to reach a nearby cell tower, such as is the case with stadiums.
A cellular distributed antenna system (DAS) is often employed within buildings and other facilities in order to facilitate transmission of signals between occupants' cell phones and local cell towers. Multiple antennas are located throughout the facility, such as on each floor. Signals to and from the distributed antennas are routed—by cable—through a central processing rack in the basement or on the first or top floor. One or more cables connects the central processing rack to an outside antenna that is pointed or otherwise configured to optimally communicate with a local cell tower. The outside antenna is often located on a building's roof.
An Emergency Responder Radio Coverage System (ERRCS) DAS may also be employed within facilities. An ERRCS DAS boosts radio signals for firemen, policemen, and other first responders, similarly to a cellular DAS.
If there is an emergency in the building, a DAS may be critical for communications. Firefighters and policemen need to communicate with one another while responding. Users should be able to communicate with the outside as well. It may be especially unnerving for users to have their otherwise-normally-operational cell phones experience an outage during a building emergency.
It is for these and other reasons that building fire codes require DASes to meet certain survivability standards. For example, building fire codes sometimes dictate that communication cables connecting the DAS's antennas to the central processing/head-end rack and communication cables running from the rack to the outside antenna maintain operation at 1010° C. (1850° F.) temperatures for two hours. This standard can be found among the NFPA 72 (National Fire Alarm and Signaling Code), ICC IFC 510 (International Fire Code), and NFPA 1221 (Standard for the Installation, Maintenance, and Use of Emergency Services Communications Systems) codes.
Yet no such fire-resistant cables exist in a coaxial configuration. To maintain spacing between the central and outer conductors, common coax cables employ dielectrics that are air-based (foam) or polymer. Plastic foam and polymers simply melt at high temperature. Therefore, in one large city with such building codes, building inspectors routinely grant waivers for DAS coax cables or require extensive fire shielding of the cables, such as boxing them in drywall soffits, to afford them the two-hour burn time.
There is a need in the art for more survivable coax communication cables.